Lower frequency signals (such as baseband signals) are often converted to higher frequencies for various applications. One example of a conversion is in the area of wireless transmitters. Typically, a baseband signal is converted to an intermediate frequency (IF) before being converted to the final output radio frequency (RF). In the wireless context, this conversion to an intermediate frequency is often necessary because (1) the required filtering may be impossible at the final output radio frequency; and/or (2) the use of an IF allows for easier and more inexpensive filtering and amplification.
Other examples of wherein a signal is translated to a higher frequency include ADSL and cable modems which modulate signals to higher frequencies.
With recent advances in digital processing speed, it is possible to create a digital baseband signal and digitally modulate such a signal, thereby creating a digital passband signal. This digital passband signal, sometimes called a digital IF, if it is not at the final output frequency, is then sent to a high speed digital to analog converter (DAC) whose output can be subsequently filtered, amplified, and further upconverted as desired. Prior art digital implementations perform this modulation by either multiplying by a sinusoid or by use of a look-up table containing results of such multiplications.
The constraint for sampling rates in digital systems, that are generally applicable, is that the sampling rate should be greater than twice the highest frequency component in the desired signal. The minimum sampling frequency is often called the xe2x80x9cNyquist rate.xe2x80x9d Prior art implementations of transmitters have required the sampling rate to be more than twice the highest frequency component of the modulated signal.
In addition, when a digital signal is converted into an analog signal, distortion may occur. One example of distortion is a multiplication of the signal by sin(x)/x. Prior art implementations have attempted to compensate for this distortion by modifying the baseband signal prior to introduction to the Digital to Analog Converter (DAC). However, particularly in instances where the baseband signal is being shifted, this implementation may be inadequate since the higher frequencies of interest are not the focus of the compensation.
This invention provides a method of translating a baseband signal to frequency higher than twice the sampling rate by delivering a lowpass signal to a DAC and then filtering out all output images except for the image at the desired frequency.
This invention further provides a novel use of a shifted sin(x)/x response to compensate for distortion from the DAC at the frequency of interest.
Therefore, an object of the invention is to provide a method and apparatus configured to translate a baseband or lowpass signal to a frequency higher than twice the sampling rate.
It is a further object of the invention to provide a method and apparatus configured to process at least one image which is output of the digital to analog converter.
It is also an object of the invention to provide a method and apparatus configured to compensate for the distortion from the digital to analog converter.
It is still a further object of the invention to provide a method and apparatus configured to shift the sin(x)/x response to compensate for distortion from the DAC at the frequency of interest.
The advantages of the present invention will become apparent to those of ordinary skill in the art by reading the following detailed description, with appropriate reference to the accompanying drawings.